More specifically, the present invention relates to a unit for packing groups of products, the unit comprising a packing wheel operating with a first movement; a feed station for feeding packing material in sheets to said packing wheel; a feed line operating with a second movement to feed packing material in the form of a continuous strip; a cutting device located along said feed line to cut the continuous strip into said sheets; and feed means for feeding said sheets successively to said feed station; said packing wheel comprising a number of seats, each for receiving a respective said product, and a number of gripping means, each associated with a respective said seat and travelling through said feed station with said first movement to receive a respective said sheet.
In known packing units, particularly packing units normally used in the tobacco industry for packing groups of packets of cigarettes, the continuous strip is normally fed continuously, the packing wheel is normally rotated in steps, and the feed means comprise an intermediate suction roller located immediately downstream from the cutting device, and which first receives the end of the continuous strip before it is cut, and then feeds a sheet, as soon as it is cut off, to the packing wheel.
Operation of the intermediate roller is therefore conditioned by both the feed movement of the continuous strip, and the rotation movement of the packing wheel, and should, theoretically, be controlled to rotate with a surface speed at least equal, at the continuous strip receiving point and instant by instant, to the travelling speed of the continuous strip, and at most equal, at the transfer point to the packing wheel and instant by instant, to the travelling speed of the packing wheel.
When, as is normally the case, for example, in packing units for packing groups of packets of cigarettes, the continuous strip and the packing wheel travel at different speeds, and the intermediate roller receives the continuous strip as it is transferring a sheet to the packing wheel, control of the intermediate roller, which is normally driven by a main motor of the packing unit and speed-controlled by cam systems, is obviously extremely difficult and not always satisfactory.